“Huge flames” were seen coming from a toilet just ahead of noon on Thursday before customers were evacuated, reportsThe Sun.

The perimeter was closed off while authorities investigated the area, and McDonald’s officials are set to inspect other restaurants.

Police have yet to confirm an explosion, but it is believed that the toilets have been destroyed. It’s been suggested that a large firecracker was responsible for the blast.

Daily Mailreports that the blast may have occurred after employees on the ground floor saw flames coming from the second floor. They immediately called police. A bomb squad was reportedly called in to investigate.

Around 30 people were reportedly in the restaurant at the time of the blast, but all were evacuated and are believed to be unharmed.

Officers from Lyon were flown in to help determine the cause of the explosion.

One of the main roads that run through the city is closed off, and the tram line is no longer running between the Louise Michel and Condorcet stations, according to The Sun. Police are asking people to avoid the area.

Officials have deemed the act a criminal one.

This story is still developing. Check back with Crime Online as additional details come through.